A method for medically employing ultrasound within a body cavity of a patient. An end effector is obtained having a medical ultrasound transducer assembly. A biocompatible hygroscopic substance is obtained having a non-expanded anhydrous state and having an expanded and fluidly-loculated hydrated state. The end effector, including the transducer assembly, and the substance in substantially its anhydrous state are inserted into a body cavity (such as endoscopically inserted into a uterus) of a patient. The transducer assembly is used to medically image and/or medically treat patient tissue (such as stopping blood flow to, and/or ablating, a uterine fibroid). A system for medically employing ultrasound includes the end effector and the substance. In another system, the end effector includes the substance. The substance in its hydrated state expands inside the body cavity providing acoustic coupling between the wall of the body cavity and the transducer assembly.